476 temperature of the Ocean. [Book VII, 



of the fpring water, it may readily be underftood, that 

 the heat of this water will not be inftantaneouily com- 

 municated through the whole mafs of water in the 

 ciftern, but that the water will decreafe in heat from the 

 furface to the bottom of the ciftern. On the other 

 hand, if on the n feet of fpring water heated to 48 

 degrees, we pour a foot of water heated only to 33 de- 

 grees, it may be expected,, that the fpring water, which 

 is nearcft to the cold water, will be fuoner cooled by 

 it than that which is at a greater diftance ; and on this 

 account the water at the bottom of the ciftern will be 

 warmer than that in the middle or at the top. It muft 

 be obierved, however, that cold water being, bulk for 

 bulk, heavier than hot water, the water which has only 

 33 degrees of heat will defcend, by its fuperior weight, 

 into the mafs of water contained in the ciftern, and 

 thus the water in the ciftern will be cooled, not only 

 by the bare communication of cold from the upper 

 water, but by 'the actual mixture of that water with the 

 reft, fo that the difference between the heat of the wa- 

 ter at the bottom and top will not be fo great as it 

 would have been if the cold water had not mixed it- 

 felf with the reft. Thefe fuppofitions of hot and cold 

 water incumbent on the fpring water in the ciftern, are 

 analogous to the aftion of the furnmer and winter at- 

 mofpheres incumbent on the furface of the fea. No 

 perfon, who has bathed in deep (landing water in fum-* 

 mer time, can have failed to obferve, that the water 

 grew colder and colder, according to the depth to 

 which he defcended. I have frequently obferved, that 

 the furface of a pool of water of two feet in depth, has 

 in a funny day, even in winter, been five degrees hotter 

 than the water at its bottom. 



' Mr. Wales defcribes the inftrument he made ufe 

 of for trying the temperature of the feu at different 



depths^ 



